Packaging laminate for a retortable packaging container

ABSTRACT

A packaging laminate ( 10 ) for a retortable packaging container of the type which is produced by fold forming and thermosealing of a web or a prefabricated blank of the packaging laminate. The packaging laminate ( 10 ) includes a base layer ( 11 ) of paper or paperboard which has been rendered hydrophobic by stock sizing. The base layer ( 11 ) is preferably rendered hydrophobic by stock sizing by means of an aqueous dispersion or emulsion of an alkyl ketene dimer or mixture of alkyl ketene dimers with a different number of carbon atoms in their respective fat tails. Most preferably, the base layer ( 11 ) is rendered hydrophobic by stock sizing by means of an aqueous dispersion or emulsion of an alkyl ketene dimer containing 18 carbon atoms in its fat tail. The quantity of alkyl ketene dimers is suitably at least approx. 0.25 weight %, preferably 0.25-0.4 weight %, most preferably approx. 0.4 weight %, calculated on the dry weight of the base layer ( 11 ).

TECHNICAL FIELD

[0001] The present invention relates to a packaging laminate comprisinga base layer of paper or paperboard for a retortable packagingcontainer.

BACKGROUND ART

[0002] A packaging laminate of the type which is described above isknown from int.al. international patent application carrying publicationnumber WO97/02140. The prior art packaging laminate has a rigid, butfoldable base or core layer of paper or paperboard and outer,liquid-tight coatings of a heat resistant plastic on both sides of thebase layer. Between the base layer and one of the two outer plasticcoatings, there may moreover be provided a gas barrier, e.g. analuminium foil, in order to impart to the packaging laminate tightnessproperties also against gases, in particular oxygen gas.

[0003] Similar packaging laminates are also described in internationalpatent applications numbers WO97/02181 and WO98/18680.

[0004] From the prior art packaging laminates, finished packagingcontainers are produced with the aid of modem packing and fillingmachines of the type which, either from a web or from prefabricatedblanks of the packaging laminate, both form, fill and seal finishedpackages. From, for example, a blank a packaging container is producedin that the blank is first formed into a tubular container capsule inthat both longitudinal edges of the blank are folded towards andpermanently united with one another by thermosealing in a so-calledoverlap joint which extends all the way between both open ends of thetubular container capsule. The one end of the container capsule isthereafter given a substantially planar end closure (bottom) by foldforming and thermosealing of adjacent end panels of the containercapsule. The thus end sealed (provided with a bottom) container capsuleis filled with the desired contents, e.g. a food, through its other,open end which, after the filling operation, is given a similarlysubstantially planar end closure (top) by a final fold forming andthermosealing of the corresponding end panels of the filled containercapsule.

[0005] It has long been known that the shelf life of a food may beextended in that the food is subjected to a heat treatment which is soextensive that bacteria, fungi and other undesirable microorganismsexisting in the food are exterminated, neutralised or reduced. In orderto achieve complete sterilisation, this heat treatment must be carriedout under such conditions that all parts of the treated food are heatedto such an elevated level and for such a lengthy period of time as arerequired to ensure a complete destruction or deactivation of themicroorganisms in the food.

[0006] A shelf-life extending heat treatment of a packed food isgenerally carried out in a retort which is an apparatus comprising aclosed treatment chamber in which both temperature and pressure may beregulated and controlled during the heat treatment. Preferably, such aheat treatment or retorting is carried out in the manner and under theconditions which are described in greater detail in international patentapplication carrying publication number WO98/16431, which is herebyincorporated as reference. Filled packaging containers are placed in thetreatment space of the retort, whereafter a gaseous heating medium, e.g.hot steam, is caused to flow in contact with the outer walls of thepackaging containers for heating the packed food to a previouslyselected treatment temperature which generally lies within the range ofbetween 70 and 130° C. The packed food is kept at this temperature for apredetermined sufficiently long time so as to ensure as good as totalthermal destruction of the microorganisms present in the food. Thetreated food is thereafter cooled with a coolant, e.g. cold water, downto a temperature close to or somewhat above the ambient temperature,i.e. 25-30° C., whereafter the treatment is discontinued and thepackaging containers are removed from the retort.

[0007] Different foods require different forms of treatment in order toachieve complete sterility by a heat treatment in a retort. For certainfoods, more extensive heat treatment is required, i.e. more elevatedtreatment temperature and/or longer treatment time than for other foodswhich only require a shorter stay time at a relatively low treatmenttemperature in order to achieve the contemplated sterility.

[0008] A packaging container of the above-described known packaginglaminate is generally sufficiently mechanically strong and stable towithstand such a relatively gentle or less extensive heat treatment,but, on the other hand, it not seldom occurs that the prior artpackaging containers are damaged or destroyed and even become totallyunusable if the heat treatment is carried out at a relatively elevatedtreatment temperature and/or during a relatively lengthy treatment timewhich may occasionally be necessary to ensure complete and totalsterility in certain types of foods.

[0009] One problem which is often observed in connection with suchextreme, severe heat treatment is that liquid or moisture tends topenetrate into the paper or paperboard layer of the packaging containerin areas where incision edges of the paper or paperboard layer are moreor less freely exposed to the gaseous heating medium, e.g. hot steam. Anexample of such a particularly moisture and heat-sensitive area of thepackaging container is along the freely exposed incision edge whichextends all way between both ends of the tubular container capsule, i.e.the top and bottom of the packaging container. This penetration ofmoisture or liquid may, in particularly serious cases, entail that thepackaging wall swells and delaminates and thereby becomes unattractiveor wholly unusable for further handling.

[0010] Previous attempts to solve the above described problem withmoisture and liquid absorbing incision edges have taken as their pointof departure to protect or cover these edges either chemically byimpregnation with a water-repellent or mechanically by slicing andrefolding, as described in international patent application carryingpublication number WO98/18680. While both of these prior art methodssolve the problem inherent in moisture and liquid penetration, theyrequire at least one extra operation and thereby associated extraequipment during the production of the packaging laminate.

[0011] One object of the present invention is therefore to obviate theabove-discussed drawbacks in connection with the prior art technology.

[0012] Another object of the present invention is to provide a packaginglaminate for a packaging container which reliably withstands a heattreatment in a retort without the risk of being damaged because ofpenetrating moisture or liquid, even if the heat treatment is carriedout at an elevated treatment temperature and/or during a lengthytreatment time in a retort

[0013] These and other objects are attained according to the presentinvention in that the packaging laminate described by way ofintroduction has been given the characterising feature as set forth inappended claim 1.

[0014] Advantages and expedient embodiments of the packaging laminateaccording to the present invention have further been given thecharacterising features as set forth in the appended subclaims.

SUMMARY OF THE INVENTION

[0015] According to the present invention, a packaging laminate is thusprovided for a retortable packaging container including a paper orpaperboard layer which has been rendered hydrophobic by stock sizing.

[0016] Since a paper or paperboard layer rendered hydrophobic by stocksizing already from the outset has sufficient tightness propertiesagainst moisture and liquid, the hydrophobic paper or paperboard layerin the packaging laminate according to the present invention requiresneither extra chemical nor mechanical agents in order to protect theexposed incision edges. Consequently, the packaging laminate accordingto the present invention may be produced in a simple manner employingalready existing conversion equipment without the employment of extraoperational steps and associated extra equipment during the productionof the packaging laminate.

[0017] The paper or paperboard layer rendered hydrophobic may, accordingto the present invention, be produced by stock sizing by means of alkylketene dimers dispersed or emulsified in aqueous solution. In practicalexamples, such aqueous dispersions or emulsions contain alkyl ketenedimers with 16-22 carbon atoms in their fat tail.

[0018] In a particularly preferred embodiment according to the presentinvention, the packaging laminate includes a paper or paperboard layerwhich has been rendered hydrophobic by means of an aqueous dispersion oremulsion of an alkyl ketene dimer which has a fat tail of 18 carbonatoms. An aqueous dispersion or emulsion of an alkyl ketene dimer with18 carbon atoms makes possible not only a high absorption of the alkylketene dimer in the paper pulp and thereby a high closure of the pores(hydrophobic) in the produced paper or paperboard layer, but thisdesired high absorption may be carried out without impeding thesubsequent handling and processing of the paper pulp in the papermachine.

[0019] Preferably, the paper or paperboard layer in the packaginglaminate according to the present invention includes an alkyl ketenedimer in a quantity which is greater than approx. 0.25 weight %,calculated on the dry weight of the paper or paperboard layer. Expressedotherwise, the quantity of alkyl ketene dimer should preferably begreater than 2.5 kg per tonne of paper or paperboard, but should not begreater than approx. 4 kg which is a maximum permitted quantity for usein food contexts.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

[0020] The present invention will be described in greater detailhereinbelow, with reference to the accompanying drawing, in which:

[0021]FIG. 1 schematically shows a cross section of a packaging laminatein its simplest embodiment according to the present invention; and

[0022]FIG. 2 shows the packaging laminate according to one preferredpractical embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0023]FIG. 1 thus schematically shows a cross section of a packaginglaminate in its simplest embodiment of the present invention. Thepackaging laminate with the generic reference numeral 10 includes a baseor core layer 11 of paper or paperboard and outer, liquid-tight coatings12 and 13 of a thermosealable plastic on both sides of the base layer11.

[0024] The base layer 11 has been rendered hydrophobic by stock sizingby means of an aqueous dispersion or emulsion of an alkyl ketene dimeror a mixture of alkyl ketene dimers of varying numbers of carbon atomsin their respective fat tails, preferably between 16 and 22 carbonatoms. In one particularly preferred embodiment, the base layer 11 hasbeen rendered hydrophobic by stock sizing by means of an aqueousdispersion or emulsion of an alkyl ketene dimer with 18 carbon atoms inits fat tail.

[0025] The quantity of alkyl ketene dimers in the base layer 11 mayvary, but is preferably at least approx. 0.25 weight % calculated on thedry weight of the base layer. In such cases where the packaging laminate10 according to the present invention is intended to be employed inconnection with a food, the quantity of alkyl ketene dimer should not,however, exceed 0.4 weight % calculated on the dry weight of the baselayer 11. Expressed otherwise, the quantity of alkyl ketene dimer ispreferably 2.5-4 kg/tonne of paper or paperboard in the dry base layer11.

[0026] The material in the two outer liquid-tight coatings of plastic 12and 13 is preferably a thermosealable plastic in order to make for arational production of finished retortable packaging containers.Examples of such thermosealable plastics for use in the two outerliquid-tight coatings 12 and 13 include, but are not limited to,polyethylene (PE), polypropylene (PP), polyester (PET) and copolymersthereof. Examples of a usable polythene plastic may be a high densitypolyethylene (HDPE) or a linear low density polyethylene (LLDPE), andexample of a usable polyester plastic may be an amorphous polyester(APET).

[0027]FIG. 2 shows a schematic cross section of a packaging laminateaccording to one preferred embodiment of the present invention. Thepackaging laminate with the generic reference numeral 20 has a baselayer 21 of paper or paperboard and outer, liquid-tight coatings 22 and23 on both sides of the base layer 21.

[0028] The packaging laminate 20 further displays a gas barrier 24between the base layer 21 and one of the two outer liquid-tight coatings22.

[0029] Between the liquid-tight coating 22 and the gas barrier 24, thereis a layer 25 of an adhesive by means of which the liquid-tight coating22 is bonded to the gas barrier 24.

[0030] Between the gas barrier 24 and the base layer 21 there is a layer26 of a lamination or sealing agent of such a nature that the gasbarrier 24 is bonded to the core layer 21 with sufficiently strong andstable bonding strength so as not to be weakened or lost entirely whenthe packaging laminate 20 is exposed to extremely elevated temperaturestresses, i.e. temperatures of up to approx. 130° C. or higher.

[0031] The outer, liquid-tight coating 23 on the other side of the baselayer 21 may display decorative art work 27 of a suitable printing inkwhich in its turn is protected by a transparent layer 28, applied abovethe decorative art work 27 and consisting of suitable lacquer or otheragent so as to protect the decorative art work 27 against attack fromoutside.

[0032] In order to impart to the packaging laminate 20 greaterintegrity, there is disposed, between the gas barrier 24 and the layer26 of lamination or sealing agent, a layer 29 of an adhesive by means ofwhich the gas barrier 24 is bonded to the layer 26 of lamination orsealing agent.

[0033] The base layer 21 has been rendered hydrophobic by stock sizingby means of an aqueous dispersion or emulsion of an alkyl ketene dimeror a mixture of alkyl ketene dimers with a varying number of carbonatoms in their respective fat tails, preferably between 16 and 22 carbonatoms. In one particularly preferred embodiment, the base layer 21 hasbeen rendered hydrophobic by stock sizing by means of an aqueousdispersion or emulsion of an alkyl ketene dimer with 18 carbon atoms inits fat tail.

[0034] The quantity of alkyl ketene dimer in the base layer 21 may vary,but is preferably at least approx. 0.25 weight % calculated on the dryweight of the base layer 21. In such cases where the packaging laminate20 according to the present invention is intended to be employed inconnection with foods, the quantity of alkyl ketene dimer should,however, not be greater than approx. 0.4 weight % calculated on the dryweight of the base layer 21. Expressed otherwise, the quantity of alkylketene dimer is preferably 2.5-4 kg/tonne of paper or paperboard in thedry base layer 21.

[0035] The outer, liquid-tight coating 23 may be a plastic which isselected from the group essentially comprising polyethylene (PE),polypropylene (PP) and polyester (PET) or mixtures thereof. Examples ofusable polyethylene plastic may be high density polyethylene (HDPE) orlinear low density polyethylene (LLDPE), an example of a usablepolypropylene plastic may be oriented polypropylene (OPP), and anexample of a usable polyester plastic may be amorphous polyester (APET).

[0036] Preferably, the liquid-tight coating 23 consists of a physical ormechanical mixture of polypropylene (PP) and polyethylene (PE) which, inaddition to superior tightness properties against liquid, also possessessufficient moisture and heat resistance to withstand such extrememoisture and temperature stresses as occur in a normal shelf-lifeextending heat treatment in a retort. An outer coating 23 of a physicalor mechanical mixture of polypropylene (PP) and polyethylene (PE)moreover possesses superior printability at the same time as makingpossible mechanically strong and liquid-tight seals byso-called-thermosealing, when the packaging laminate 20 is reformed intoa retortable packaging container.

[0037] In the same manner, the outer, liquid-tight coating 22 mayconsist of a plastic which is selected from the group essentiallycomprising polyethylene (PE), polypropylene (PP), polyester (PET) andcopolymers thereof. An example of a usable polyethylene plastic may be ahigh density polyethylene (HDPE) or a linear low density polyethylene(LLDPE), and an example of a usable polyester plastic may be anamorphous polyester (APET).

[0038] Preferably, the liquid-tight coating 22 consists of a copolymerof propylene and ethylene which is sufficiently moisture and heatresistant to withstand extreme moisture and temperature stresses whichoccur in a normal shelf-life extending heat treatment in a retort. Acoating of a copolymer of propylene and ethylene moreover makes formechanically strong and liquid-tights seals when the packaging laminate20 is reformed into a retortable packaging container.

[0039] The layer 24 serving as gas barrier may consist of an organic oran inorganic material. An example of a usable inorganic material may bea metal foil, e.g. an aluminium foil, or a silica oxide coating producedby plasma deposition, and an example of a suitable organic material maybe a so-called barrier polymer, e.g. a copolymer of ethylene and vinylalcohol (EVOH).

[0040] Preferably, the gas barrier 24 is an aluminium foil which, inaddition to superior tightness properties against gases, in particularoxygen gas, also makes for sealing of the packaging laminate 20 byinduction thermosealing which is a simple, but rapid and efficientsealing technique.

[0041] From a packaging laminate according to the present invention,well-functioning retortable packaging containers are produced by foldforming and thermosealing in the previously described manner, packagingcontainers which may be reliably heat treated at extremely elevatedmoisture and temperature conditions in a retort, without the packaginglaminate delaminating because of moisture and steam which are preventedeffectively from penetrating into the base layer, which has beenrendered hydrophobic, through exposed incision edges.

What is claimed is:
 1. A packaging laminate for a retortable packagingcontainer of the type which is produced by fold forming and sealing of asheet or web-shaped blank of the packaging laminate, the packaginglaminate (10; 20) including a base layer (11; 21) of paper or paperboardand outer, liquid-tight coatings (12, 13; 22, 23) of thermosealableplastic on both sides of the plastic layer (11; 21), characterised inthat the paper or paperboard layer is rendered hydrophobic by so-calledstock sizing by means of an aqueous suspension or emulsion of an alkylketene dimer or mixture of alkyl ketene dimers.
 2. The packaginglaminate as claimed in claim 1, characterised in that the paper orpaperboard layer (11; 21) contains said alkyl ketene dimers in aquantity of at least 0.25 weight %, calculated on the dry weight of thepaper or paperboard layer (11; 21).
 3. The packaging laminate as claimedin claim 1 or 2, characterised in that the paper or paperboard layer(11; 21) contains between 2.5 and 4 kg of alkyl ketene dimers per tonneof paper or paperboard in the dry paper or paperboard layer (11; 21). 4.The packaging laminate as claimed in any of the preceding claims,characterised in that said alkyl ketene dimers are selected from thegroup essentially comprising alkyl ketene dimers with 16-22 carbon atomsin their respective fat tails.
 5. The packaging laminate as claimed inclaim 4, characterised in that said alkyl ketene dimers have 18 carbonatoms in their fat tail.
 6. Use of a paper or paperboard layer (11; 21)rendered hydrophobic as base layer in a packaging laminate (10; 20) asclaimed in any of the preceding claims.
 7. A retortable packagingcontainer of a packaging laminate comprising a core layer (11; 21) ofpaper or paperboard and outer, liquid-tight coatings (12, 13; 22, 23) ofthermosealable plastic on both sides of the base layer (11; 21),characterised in that the paper or paperboard layer (11; 21) is renderedhydrophobic by so-called stock sizing by means an aqueous suspension oremulsion of at least one alkyl ketene dimer.